EVENT: On the role of the ambient fluid on gravitational granular flow dynamics

Gravitational granular flows are common in nature. Typical geophysical examples include debris avalanches, pyroclastic flows, landslides, cliff collapses, and submarine avalanches. The dynamics of these flows can be studied by considering the particular dynamics of at least two constituents, the ambient viscous fluid and the solid phase. Therefore, in order to describe the dynamics of the solid phase, the role of the ambient fluid should be taken into account. In spite of this, the effects of the ambient fluid on granular flow dynamics are poorly understood and commonly ignored in analyses.

In this research, we characterised and quantified these effects by combining theoretical and experimental analysis. Starting with the mixture theory, we derived a set of two-phase continuum equations for studying a compressible granular flow composed of homogenous solid particles and a Newtonian ambient fluid. The role of the ambient fluid was then investigated by studying the collapse and spreading of two-dimensional granular columns in air or in water, for different solid particle sizes and column aspect (height to length) ratios, in which the front speed was used to describe the flow. The combined analysis of experimental measurements and numerical solutions showed that the dynamics of the solid phase cannot be explained if the hydrodynamic fluid pressure and the drag interactions are not included in the analysis. For instance, hydrodynamic fluid pressure can hold the reduced weight of the solids, thus inducing a transition from dense-compacted to dense-suspended granular flows, whereas drag forces counteract the solids movement, especially within the near-wall viscous layer. We concluded that in order to obtain a realistic representation of gravitational granular flow dynamics, the ambient fluid cannot be neglected.